The invention concerns a process for measuring a weft or mesh serial position in textiles, and an apparatus for carrying out said process.
In the production of textile fabrics, warp and weft yarns cross each other precisely at right angles. However, during different subsequent working cycles in the equipment, the fabrics can be distorted. In the production of meshed articles in circular knitting machines, the resulting tubular work is cut open so that the meshed article has a general diagonal distortion after being cut open. In both cases, such distortion must be prevented by adequate straightening machines which use the distortion angle as a control value. Therefore, it is necessary to measure the distortion angle.
An apparatus is known for measuring the distortion angle, in which a source of light radiates the continuous textile webs. Two photocells with apertured stops situated in front thereof, are disposed opposite to the source of heights, the central axes of the apertures forming angles with each other. A measure of the angular course of the weft yarn can be deduced from the differential signal of the photocells. However, the use of two optical electric systems is disadvantageous because of the balance problems involved and of the small "pull-in range" determined by the angle between both apertured stops.
In German Pat. No. 16 35 266 there is described an apparatus in which a single aperture with photosensors situated at the rear thereof, is reciprocally rotated about an angle by an electrodynamic drive system, the rotational movement taking place about a central angle determined by the proximity to the mechanical resonance frequency of the system. Therefore, the speed of the rotational movement is predetermined by the system. The output signal of the photosensor is summed over a period of time by an amplifier, the sign of the amplification being always reversed on opposite sides of the line defining the central angle. The signal summed over a period is therefore zero when the measured values are symmetrically distributed around the central angle. This is the case when the weft yarn has the same direction as the central angle. In addition, such a system includes a follower control device which adjusts the entire system or the central angle according to a momentary measurement in a manner such that the central angle always runs parallel with the weft. Therefore, a direct measuring of the course of the weft or of the distortion angle is possible with reference to the central angle.
This known system is disadvantageous in that it is not possible by the mechanically predetermined oscillation frequency, to coordinate the measuring with the speed of the fabric or with the number of weft yarns per unit time. In case of high speeds of the fabric or when many weft yarns per unit time are passed by the apparatus, the measuring speed is limited by the oscillation frequency. In the case of low speeds of the fabric and/or very thick weft yarns (such as carpets or the like), the oscillation frequency is much too high so that it is no longer possible to obtain any correct measurement of the distortion angle. Further, due to the manner in which the measurement results are evaluated that is, reversal of the central angle, there is the danger that "secondary maximums" that occur in certain kinds of fabrics and reach into the pull-in range may be tracked. As a result, the follower control device may not track the course of the weft yarn, but instead, may track a pattern moving diagonally thereto and determined by the kind of weave.
German Pat. No. 11 09 636 has disclosed a similar apparatus wherein, to the contrary, the linear range does not reciprocally oscillate, but rather, rotates about a central angle.
Although a better adaptation of the speed of the angular variation to the speed of the angular variation to the speed of the fabric is possible with this apparatus, the largest part of information is lost, since the aperture moves only over a fragment of the whole circle into the angular range of interest.